Human mast cells in the neurohormonal network: expression of POMC, detection of precursor proteases, and evidence for IgE-dependent secretion of alpha-MSH.

Human mast cells have been shown to release histamine in response to the neuropeptide alpha-melanocyte-stimulating hormone (alpha-MSH), but it is unknown whether these cells express proopiomelanocortin (POMC) or POMC-derived peptides. We therefore examined highly purified human skin mast cells and a leukemic mast cell line-1 (HMC-1) for their ability to express POMC and members of the prohormone convertase (PC) family known to process POMC. Furthermore, we investigated whether these cells store and secrete alpha-MSH. Reverse transcriptase-PCR (RT-PCR) analysis revealed that both skin mast cells and HMC-1 cells express POMC mRNA and protein. Expression of the POMC gene at the RNA level in HMC-1 cells could be confirmed by Northern blotting. Transcripts for both PC1 and furin convertase were detectable in skin-derived mast cells and HMC-1 cells, as shown by RT-PCR. In contrast, PC2 transcripts were detected only in skin mast cells, whereas transcripts for paired basic amino acid converting enzyme 4 (PACE4) were present only in HMC-1 cells. Radioimmunoassays performed on cell lysates and cell culture supernatants from human skin-derived mast cells disclosed immunoreactive amounts of alpha-MSH in both fractions. Stimulation with an anti-IgE antibody significantly reduced intracellular alpha-MSH and increased extracellular levels, indicating IgE-mediated secretion of this neuropeptide. Our findings show that human mast cells are active players in the cutaneous POMC system. Mast cell-derived alpha-MSH may contribute to cutaneous hyperpigmentation as seen in patients with urticaria pigmentosa. Moreover, IgE-dependent release of alpha-MSH suggests an immunomodulatory role of this neurohormone during inflammatory and allergic reactions of the skin.